The present disclosure generally relates to electrode compositions. More specifically, the present disclosure relates to compositions for use as positive electrode materials in energy storage devices. The disclosure also includes energy storage devices that utilize such electrode compositions.
Metal halide batteries are widely employed for energy storage applications. In particular, metal chloride batteries including a molten sodium negative electrode (anode) and a beta-alumina solid electrolyte, are of considerable interest for use in industrial vehicles, telecommunication, utility applications, and uninterruptible power supply (UPS) devices. In addition to the anode, the batteries include a positive electrode (cathode) that supplies/receives electrons during the charge/discharge of the battery. The positive electrode of such batteries is usually built from nickel metal, sodium chloride (NaCl) and a molten-salt electrolyte such as sodium tetrachloroaluminate NaAlCl4.
Current development of the sodium-metal chloride batteries is focused on the improvement of the performance and the cycle life. When these metal halide batteries are employed in mobile and utility applications, such as hybrid locomotives or plug-in electric vehicles (PHEV), the battery should tolerate power surges (high currents) during both charging and discharging, without a loss in the capacity and the cycle life. Generally, when these batteries are discharging using high discharge current rates (e.g., at 110 W/cell for a 110 W-h cell), multiple discharge cycles may be conducted with no significant increase in the resistance, increase in the charging time, or loss of the capacity. However, when discharging at low discharge currents, the sodium metal halide batteries may degrade very rapidly, leading to low charging rate (i.e. the charging time may increase).
A common way to improve the performance of these batteries is an addition of a small amount of various additives to the positive electrode composition. The use of sodium salts of other halogens (NaF, NaBr and NaI), and/or elemental sulfur as additives has been tried. Addition of iron monosulfide (FeS) instead of elemental sulfur allowed for better sulfur distribution in the electrode material and less variability. High amounts of sulfur in the positive electrode (U.S. Pub. No. 20140178791A1) have shown an improvement in the charging rate and a reduction in the degradation rate. These attributes improve the performance of a battery, as compared to one having a small or negligible amount of sulfur in the positive electrode.
There continues to be a growing need for additional improvements in the performance and the cycle life of the batteries. It may be desirable to have an electrode material that maintains or improves the performance of a sodium-metal chloride battery.